Filters - Specifications & Selections

Stand-alone passive noise filters have specific limited uses, and are not often purchased by end users. Their design and application considerations are so complicated that they should be used only as advised by trained power quality professionals. Nonetheless, be aware of the major considerations in selecting and installing them.

Filters are selected based on the type of noise that needs to be eliminated, as well as the type and size of the load. The filter is sized to match the service voltage and current requirements. It must allow the fundamental 60 Hz voltage to pass through, while eliminating all higher frequencies.

It is important to note that not all electrical noise comes from power lines. Some is introduced into equipment from radio frequencies, and power line noise filters will do nothing to eliminate this. Passive filters must be installed in the line between the source voltage and the affected equipment. Small filters are usually simple plug-in devices, while larger filters must be hard-wired into the system by an electrician.

In selecting equipment, experts look for devices that have the following characteristics:

  • Noise reduction of 60 dB or greater,
  • Common mode noise attenuation from 10 kHz to 1 00 kHz,
  • An efficiency of at least 90%, and
  • A manufacturer's warranty of at least one year.

Another important point is that while passive noise filters can attenuate some common mode noise, they will not eliminate all such noise. Common mode noise occurs between the neutral and ground, and will~often cause local area networks to malfunction. An isolation transformer is used to resolve these problems. Like all transformers, these are sized to match their connected load and service voltage. They are wired between the source voltage and the affected load. Some of the smaller isolation transformers are plug-in devices, while the larger units require installation by an electrician.

Installing harmonic filters involves many of the same considerations. Harmonic filters are selected to match the particular harmonic current that needs to be eliminated. If not, the filter won't work. To match the filter to the current, a supplier or a reputable contractor will measure the harmonic content on the system. Most suppliers will require this measurement to properly install a filter. Never purchase or install harmonic filters over the counter. The utility may be able to help a customer find a qualified supplier of harmonic measurement and mitigation equipment.